The Investigations Of Strain Induced Multiferroic,Dielectric Properties And Magnetoelectric Coupling In Paraelectric Eu TiO₃ Material

As one type of novel multifunction materials, in recent years, multiferroics have received a great deal of attentions because of their properties exhibiting electric, magnetic, and elastic order in one phase, which may open up a new path to design novel promising electronic devices in the field of information storage, magnetic sensor, spintronics design and so on. However, the previous studies have indicated that multiferroics very few exist in nature and a large part of them are in antiferromagnetic(AFM)-ferroelectric(FE) state which has no benefit of practical application. So the issues about how to develop FM-FE materials, and how to achieve the magnetoelectric regulation and unique dielectric optical properties in multiferroics are becoming the research hotspots.In this thesis, a first-principles calculation is performed based on projection augmented plane wave method(PAW) of density functional theory(DFT) with generalized gradient approximation(GGA) to study the change of magnetic, ferroelectricity and optical properties in EuTiO3 induced by the biaxial strains, and investigate the magnetic phase transition, spin polarization flop, magnetoelectric coupling and optical anisotropy, because of abundant electrical properties, magnetic properties and instability of quantum paraelectric EuTiO3 material in ground state. The main contents include as followings: In chapter 1, a brief introduction which contains basic concepts, history and present research situation of multiferroics materials is given. Moreover on the basis of the introduction a summary of existing problems is listed. In chapter 2, some points of knowledge about quantum mechanics and density functional theory(DFT) are been listed to illustrate the Vienna ab initio simulation package(VASP). In chapter 3, a mechanism explanation of the magnetic phase transition as well as spin polarization flop, and a brief analysis of dielectric anisotropy induced by strain are given by optimizing the crystal structure, contrasting the energy band and analysising the density of states(DOS) in different strain and magnetic state of selected EuTiO3 supercell. In chapter 4, the major results of the work are summarized, and a prospect is given in the field of experiment and theory according to a part of the results.In this work, 36 kinds of supercell with different crystal structures and magnetic structure are set for our first-principle calculations, because four kinds of magnetic configurations(AAFM, CAFM, FM and GAFM) and three different directions of magnetic moments([100], [001] and [110] direction) are taken into consideration, respectively, in paraelectric and biaxial compressive and tensile strain induced ferroelectric phase. Then, The main innovative achievements are as follows:The phenomenon of spin polarization flop accompanying the magnetic phase transition is revealed by the non-collinear magnetic structure calculations by taking magnetic anisotropy into consideration. By comparing the site-projected partial density of states(PDOS) of Eu 4f orbits under each the magnetic structure, we conclude the strain induced spin reorientation seems to should be determined by the orbits with the most localized DOS; By comparing the energies of the unit cell connected with the crystal structure and the magnetic structure individually under compressive strain, the phenomenon of that the structural inversion of the EuTiO3 with asymmetric structure in the FE phase will induce magnetization reversal is also revealed, so it would provide a theoretical basis for the realization of electrically controlled magnetic; On the basis of the Eu-O-Eu super exchange path besides the Anderson super exchange model, we obtain the values of the magnetic exchange interaction, and give a consummate explanation of theoretical mechanism for the strain induced EuTiO3 magnetic phase transition accordance to the changes of the super exchange path bond angles; We investigate the change of the dielectric function in the system under compressive strain, and the spectrum indicate that compressive strain not only induce the dielectric anisotropy, but also makes the optical absorption shifted to shorter wavelength.